Centrifugal filters are widely used for solid-liquid separation for a variety of particulate materials. In the coal and minerals industry, one type of particulate material is separated from another using various solid-solid separation methods. Since the separation is usually carried out in aqueous media, it is necessary to dewater the products before shipping to customers or downstream processes. In the coal industry, basket centrifuges are used to dewater the particles that are larger than approximately 1 mm, while finer particles are dewatered by means of screen bowl centrifuges. The latter is capable of providing considerably lower moistures than the more traditional vacuum filters, partly due to the loss of finer particles as effluent during filtration. In general, the moisture of dewatered product increases with decreasing particle size due to increased surface area. Therefore, elimination of the finest particles as effluent should help lower the dewatered product; however, it entails loss of valuables, which is not desirable.
When an aqueous suspension of particles is introduced to a batch centrifuge whose wall is made of a porous medium, the heavier solids settle quickly on the medium while the lighter water form a layer over the cake. As centrifugation continues, water begins to flow through the cake. The initial dewatering process, in which water flows through the cake while the cake is covered with a layer of water, is referred to as filtration. In time, the layer of water disappears from the surface of the cake, and the capillaries in the cake become saturated with water. The dewatering process that occurs with no water over the cake is referred to as drainage. For the reasons given below, the drainage process is much slower than the filtration process. Control of the rate of drainage is critical in controlling the final cake moisture.
The rate of drainage through the cake can be predicted by Darcy""s law:                     Q        =                              K            ⁢                          xe2x80x83                        ⁢            Δ            ⁢                          xe2x80x83                        ⁢            PA                                μ            ⁢                          xe2x80x83                        ⁢            L                                              [        1        ]            
where Q is the flow rate, K the permeability of the cake, xcex94P the pressure drop across the cake, A the filtration area, xcexc the dynamic viscosity of water, and L is the cake thickness. During the filtration period, the pressure drop across the cake is determined by the following relationship:                                           Δ            ⁢                          xe2x80x83                        ⁢            P                    =                                    1              2                        ⁢                                          ρω                2                            ⁡                              (                                                      r                    S                    2                                    -                                      r                    0                    2                                                  )                                                    ,                            [        2        ]            
where xcfx81 is the density of the liquid, xcfx89 the angular velocity, and r0 and rs are the radial distances of the free water and the cake surface from the rotational axis of a centrifuge, respectively. From Eqs. [1] and [2], one can see that the rate of filtration should increase with xcfx89 and the thickness (rsxe2x88x92r0) of the water over a filter cake.
According to Eq. [2], xcex94P becomes zero, when the water over the cake disappears, i.e., r0=rs. As the water level in the cake decreases further, i.e., r0 greater than rs, the pressure within the cake becomes lower than the ambient pressure, as shown by the mathematical model developed by Zeitsch (in Solid-liquid Separation, 3rd Edition, edited by L. Svarovsky, Buttreworth, London, 1990, p.476). The model calculations show that the pressure in the cake becomes increasingly negative with increasing cake thickness.
Despite the lack of positive pressure drop in the cake, dewatering occurs during the drainage period inasmuch as the centrifugal force within the cake exceeds the sum of the forces holding the water in the capillaries, the forces created by the negative pressure, and the forces due to hydrodynamic drag. The process of drainage relying solely on the centrifugal force entails high energy consumption and requires high maintenance to obtain low cake moistures. Energy consumption and maintenance are the major concerns in using centrifugal filters for solid-liquid separation. In the present invention, methods of overcoming these problems are disclosed. They include methods of increasing the gas pressure inside a centrifuge and/or reducing the air pressure outside. These provisions are designed to increase the pressure drop across a filter cake, so that one can take advantage of the Darcy""s law (Eq. [1]), which suggests that dewatering rate should increase with increasing pressure drop. The extraneous methods of increasing the pressure drop, as disclosed in the present invention, is particularly useful for increasing the rate of dewatering during the drainage period, which is critical in achieving lower cake moistures. The methods disclosed in the present invention are useful for obtaining low cake moistures without causing high energy consumption and maintenance problems.
A series of U.S. patents (U.S. Pat. Nos. 3,943,056 and 4,052,303) awarded to Hultch disclosed a method of creating a negative pressure on the outside wall of a centrifuge and thereby increasing filtration rate. This is accomplished by creating a chamber outside the filter medium, in which filtrate water is collected. Since the water in this chamber is subjected to a larger centrifugal force that that remaining in the cake, a negative (or vacuum) pressure is created due to a siphon effect. This technique is, therefore, referred to as the method of using rotating siphon. However, the effectiveness of this method breaks down as soon as air enters the filtrate chamber through the filter cake. This will not allow a sufficiently long drainage period, which is often necessary for producing low cake moistures.
The U.S. Pat. No. 4,997,575 teaches a method of using rotating siphons in a pressure housing with superatmospheric pressure, which is controlled by a difference in filtrate liquid levels in the filtrate liquid chamber and the annular space following the filter. This liquid control prevents the penetration of filtrate liquid into the gas exhaust line.
The U.S. Pat. Nos. 5,771,601 and 5,956,854 teach a method of injecting a gas stream such as air into the bed of particles during centrifugation and thereby reducing the surface moisture of the particles. The turbulent flow created by the gas flow strips the water from the surface of the particles. This technique is useful for the particles in the range of 0.5 to 30 mm that are dewatered in basket centrifuges. In this invention, the stream of gas is injected into an open space. Therefore, it cannot significantly increase the pressure drop across the bed of particles. Also, it would be difficult to increase the pressure drop, when a cake is continually disturbed by a scrawl, which is widely used to move the particles in basket centrifuges. Furthermore, the airflow is created by a blower rather than a compressor, which should make it difficult to create a high pressure drop across a filter cake.
According to the theoretical considerations given above, the rate of dewatering is low during the drainage period of a centrifugal filtration process, which in turn can be attributed to the lack of positive pressure drop across filter cake. This problem can be overcome by increasing the pressure drop using extraneous means such increasing the gas pressure inside a centrifugal filter and/or reducing the pressure of the gas (air) outside. It has been found that these provisions greatly enhance the rate of drainage and, thereby, lower the cake moistures.
In effect, the present invention suggests methods of combining the conventional centrifugal filtration with pressure and/or vacuum filtration. However, the moisture reductions that can be achieved using the combined method are substantially lower than the sum of the moisture reductions achieved using the different dewatering methods individually. Thus, the combined method exhibits synergism. Although the increase in drainage rate induced by the extraneous means of increasing the pressure drop can provide an explanation for the observed improvement, there may be other mechanisms that are responsible for the synergism.
In a typical operation, a slurry is introduced to a basket-type centrifuge whose side wall is made of a porous medium (e.g., screen, sintered glass, sintered ceramic, sintered metal, or filter cloth laid over screen). The top and bottom of the centrifuge is made of solid material(s) so that the air introduced into the centrifugal filter vessel can exit only through the porous side wall. The centrifuge can be positioned vertically, horizontally, upside down, or with any angle, as the gravitational force is insignificantly small as compared to the centrifugal force. The feed slurry can be introduced either as dilute suspension or thickened slurry.
The centrifuge can be operated either as a batch or continuous solid-liquid separation unit. In a batch operation, the particles in the slurry quickly form a cake over the porous medium and the liquid (water) passes through the cake. The rate of the water flowing through the cake is high when the cake is covered by a layer of water, as the pressure drop across the cake is positive in accordance with Eq. [2]. As the water layer disappears from the cake surface, i.e., rs=r0, the pressure drop becomes zero, which will cause a decrease in drainage rate. The water will continue to flow through the cake under these conditions inasmuch as the centrifugal force in the cake exceeds the sum of the capillary force that holds the water on the capillary wall and the hydrodynamic drag force. The provisions of the present invention, i.e., increase in the pressure drop by the extraneous means, can increase the rate of drainage and, hence, lower the cake moisture.
In one embodiment of the present invention, the pressure inside a centrifugal filter vessel is increased by introducing a stream of compressed air. This will increase the pressure drop across the filter cake and, hence, the rates of both filtration and drainage. The real advantage of using the compressed air is found during the drainage period. As has already been noted, the pressure inside a cake becomes zero or negative depending on the cake thickness and angular velocity. The applied air pressure will provide a net positive pressure drop, which should greatly increase the rate of drainage and lower the final cake moisture.
Another embodiment of the present invention is to increase the pressure drop across filter cake by applying a vacuum pressure on the outside wall of the centrifugal filter described above.
Still another embodiment of the present invention is to apply compressed air inside a centrifugal filter vessel and at the same time apply a vacuum on the outside. However, this method may be reserved only for the cases of dewatering materials that are very difficult to treat. The method of using either compressed air or vacuum pressure alone may be sufficient for dewatering many coal and mineral fines, as will be shown in the examples given in this invention disclosure.
Yet another embodiment of the present invention is to increase the hydrophobicity of particulate materials to increase the rate of drainage during centrifugal dewatering. According to the Laplace equation, an increase in hydrophobicity should result in a decrease in capillary pressure, which should help increase the drainage rate. This is particularly important for difficult-to-dewater materials such as precipitated calcium carbonate (PCC).
The method of increasing the pressure drop across the cake using the extraneous methods as described in the present invention has advantages over the method of using the rotating siphons in that the increased pressure drop persists during the entire drainage period. On the contrary, the method of using rotating siphons stops working as soon as the air passes through the cake. It is generally regarded that a filter cake consists of capillaries of different radii. The water in larger capillaries are more readily removed than that in smaller capillaries. Therefore, air can pass through a cake very quickly through the large capillaries and nullify the pressure drop created by the rotating siphons. This will make it difficult to remove the water in smaller capillaries. On the other hand, the method of applying air pressure or vacuum pressure as disclosed in the present invention is effective during the entire period of drainage period employed. This will give opportunities for the water trapped in smaller capillaries to be removed, which will result in low cake moistures.